The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with spiral volume imaging with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other types of volume imaging, with multiple single slice images, continuous rotating x-ray source images, gated imaging, and the like.
In spiral or helical scanning, the x-ray source or tube rotates continuously as the patient support table moves at a constant, linear velocity. In this manner, the collected data effectively represents a helical path of constant pitch through the patient. Conventionally, the data is stored and handled as a series of parallel planes, transverse to the longitudinal axis of the patient, or more specifically, as a three dimensional rectangular matrix of memory cells. See, for example, U.S. Pat. No. 3,432,657 to Slavin.
For some medical diagnoses, it is advantages to inject a patient with a contrast agent, usually a high-Z contrast material such as iodine. Depending on how far the imaged region is from the heart, the contrast material maintains its peak concentration level in the region of interest over a period of about 30-90 seconds. CT scanning techniques have been developed for generating images of the region of interest while the contrast agent is near its peak. For example, ultra fast CT, i.e. electron beam scanning, has been used to cover a significant volume. See "Power-Injected CT Contrast Opacifies Vascular Spaces", Sam D. Lane, Diagnostic Imaging, November 1988. However, this technique cannot show contrast variations over a significant volume over a period of time. Lane must leave the couch in one position in order to show contrast variations of the same region over time. Thus, the Lane technique is disadvantageous due to the increased dose of contrast agent and the limiting of measurements to only a single observed axial slice.
A volume helical scanning technique is discussed in U.S. Pat. No. 4,789,929 of Nishimura, et al. which utilizes back and forth motion of the couch to achieve more complete sampling of the volume being scanned. Nishimura does not address the concept in a temporal dimension. Moreover, the back and forth movement during data collection results in a temporally non-uniform sampling, particularly for regions of interest near the ends of the volume.
Spiral volume scans have been performed over a single patient breath-hold in the presence of a contrast agent. See "Spiral Volumetric CT with Single-Breath-Hold Technique, Continuous Transport, and Continuous Scanner Rotation", Radiology, Vol 176, pp 181-183, 1990 and "Physical Performance Characteristics of Spiral CT Scanning", Med. Phys. Vol. 18, No. 5, September/October 1991, both by Kalender, et al. One drawback of these techniques is that they use a linear interpolator for the helical interpolation, which reduces sharpness of the CT slice definition. Another drawback is that only one set of measurements is obtained from a single breath-hold.
In order to fit the spiral collected data into a conventional three dimensional rectangular matrix, a series of parallel planes are defined through the spiral collected data, with a one plane per spiral revolution, e.g. at each 0.degree. of source rotation. During the data collection period, a series of views or fans of data are collected at preselected angular increments around the patient. Potentially, one view per plane, by convention the 0.degree. or 12 o'clock view, falls squarely in the plane requiring no averaging or weighting. For each remaining view of the plane, there is a pair of corresponding views or data fans, one from the revolution preceding the plane and the other from the revolution following the plane. These views are averaged or weighted in accordance with their relative distance from the plane. In this manner, a full set of weighted views is created to perform a conventional 360.degree. CT reconstruction algorithm. See U.S. Pat. No. 4,630,202 issued December 1986 to Mori.
One of the problems with the linear interpolation technique is that it introduces errors in fourth generation scanners using source fan reconstruction. In a third generation scanner in which the x-ray source and an arc of detectors rotate together about the slice, each data fan or view is collected instantaneously in a plane parallel to the artificially defined transverse slices. In a fourth generation scanner, there is a parallel ring of stationary detectors surrounding the patient. With source fan reconstruction, each detector is sampled at monitored, time displaced intervals generating a view or fan of data as the source rotates behind the examination region. Because the patient moves longitudinally between the first and last data sampling of the view or data fan, the views are warped or canted along the spiral path. The linear interpolation scheme which assumes that the views lie parallel to the artificially defined planes introduces errors.
Another problem with the linear interpolation technique is that it is particularly sensitive to variations in the x-ray rotation speed, the velocity with which the patient is moved, and fluctuations in the output of the x-ray tube.
The present invention provides a new and improved spiral volume imaging technique.